This invention relates to optical sensors, and particularly to room temperature infrared sensors especially applicable to miniature robots ("gnat robots").
To operate properly, a robot most often requires sensing of its proximity. Therefore, known robot designs use sonar rangefinders, active near-infrared proximity sensors or laser light-striper imaging systems to endow them the capability of roaming freely through their environments, avoiding obstacles, and searching for specified landmarks or goals. These proximity sensors are active--they emit energy to the environment.
Known passive cameras are made from solid-state charge coupled devices. These devices are sensitive to visible light, and are commonly used by vision researchers to develop algorithms for image understanding. In robotics, visible light vision systems have had success, but have not achieved full image recognition owing to the incredible complexity of everyday visible-light scenes where objects have shadows, are obscured by other objects, and may blend into the background. Human eyesight is clearly our most complex sense, requiring close to half of our actual brain volume to function. Furthermore, our eyes require high bandwidth to the brain even after greatly preprocessing the visual information in the retina. Instilling human level perception into a robot is a very difficult task.
Infrared imaging, on the other hand, promises much better image recognition for certain objects while requiring only a fraction of the computing power of visible light vision. Edges in an infrared image will be primarily outlines of a single body, free from any extraneous edges due to texture or optical patterns. Most objects, and especially animate objects, tend to have a characteristic temperature which is invariant and distinct from the background temperature in nearly all conditions. Imaging systems that are sensitive to temperature can thus easily spot these objects and recognize them from their temperature.
Typical silicon infrared imagers, such as charge coupled device sensors designed for long wavelength electromagnetic energy, need to be cooled to the temperature of liquid nitrogen. This is because long wavelength photons have less energy than visible light, and the signals detected from these photons are below the CCD noise floor unless the sensor is cooled.
Pyroelectric substances have a polarized crystalline structure in which changes magnitude (inducing charge) when exposed to heat (i.e. infrared radiation). Pyroelectric sensors, therefore, can detect infrared radiation. Silicon sensors, by contrast, create charge carriers when hit by long wavelength photons. The main advantage of a pyroelectric sensor is that it does not need to be cooled.
Of the thirty-two different crystal classes, twenty-one have lattice formations with an inherent asymmetry. Twenty of those twenty-one crystals exhibit piezoelectric properties, which means that application of a voltage across the material causes a mechanical deformation and, conversely, stressing the material produces an electrical signal.
Of the twenty substances that have piezoelectric attributes, ten contain an electric dipole moment in the unstrained condition, which leads to pyroelectric characteristics. A pyroelectric material creates an electrical signal when the crystal is exposed to a change in temperature. Some of the ten pyroelectric materials also display ferroelectric traits. A ferroelectric material can have its polarization dipole reoriented in direction through the application of a strong electric field. After the electric field is removed, the crystal retains the polarization direction, effectively acting as a solid-state switch. Ferroelectrics, then, being a subset of pyroelectrics and piezoelectrics, contain all the attributes of all three. In addition, ferroelectric materials are characterized by having very high dielectric constants.
Because of their heat sensitivity, pyroelectric or ferroelectric materials may be used to sense infrared energy. Pyroelectric infrared imaging array cameras are commercially available, but are expensive. The cost of these cameras stems from a complex manufacturing technique The ceramic crystals for the pyroelectric arrays are assembled by hand, ground down manually to 20 .mu.m thickness, diced into small cells, and bump-mounted onto a hybrid substrate.
Typical pyroelectric camera readout circuitry senses the current (the change in charge) produced by the ceramic crystals. However, in a pyroelectric, a change in charge is produced by change in temperature, hence a pyroelectric camera needs relative motion of the infrared image to produce any signal. To solve this problem, a mechanical chopper is placed in front of the pyroelectric sensor to artificially produce temperature changes in static images.